Metal-based particle assembly

ABSTRACT

There is provided a metal-based particle assembly comprising 30 or more metal-based particles separated from each other and disposed in two dimensions, the metal-based particles having an average particle diameter in a range of from 200 to 1600 nm, an average height in a range of from 55 to 500 nm, and an aspect ratio, as defined by a ratio of the average particle diameter to the average height, in a range of from 1 to 8, wherein the metal-based particles are disposed such that an average distance between adjacent metal-based particles may be in a range of from 1 to 150 nm. This metal-based particle assembly presents significantly intense plasmon resonance and also allows plasmon resonance to have an effect over a range extended to a significantly large distance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2012/058597 filed Mar. 30, 2012, claiming priority based onJapanese Patent Application Nos. 2011-079474 filed Mar. 31, 2011,2011-245335 filed Nov. 9, 2011, and 2012-018645 filed Jan. 31, 2012, thecontents of all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a metal-based particle assembly that isa plasmonic material useful for light emitting devices (organicelectroluminescence (EL) devices, inorganic EL devices, inorganic lightemitting diode (LED) devices, and the like) to provide improved luminousefficiency and for photoelectric conversion devices (solar cell devices)to provide improved conversion efficiency.

BACKGROUND ART

It has conventionally been known that making metal particles small to benano-sized presents functions that are not observed when it is in a bulkstate, and localized plasmon resonance is in particular expected forapplication. Plasmon is a compressional wave of free electrons thatarises by collective oscillation of the free electrons in a metallicnanostructure.

In recent years, a field of art handling the plasmon is referred to asplasmonics and attracts large attention, and has also been activelystudied and such study includes exploiting phenomena of localizedplasmon resonance of a metal nanoparticle to be intended forimprovements of light emitting devices in luminous efficiency andimprovements of photoelectric conversion devices (solar cell devices) inconversion efficiency.

Japanese Patent Laying-Open Nos. 2007-139540 (PTD 1) and 08-271431 (PTD2) and WO2005/033335 (PTD 3), for example, disclose techniquesexploiting localized plasmon resonance for enhanced fluorescence.Furthermore, T. Fukuura and M. Kawasaki, “Long Range Enhancement ofMolecular Fluorescence by Closely Packed Submicro-scale Ag Islands”,e-Journal of Surface Science and Nanotechnology, 2009, 7, 653 (NPD 1)indicates a study on localized plasmon resonance of silvernanoparticles.

CITATION LIST Patent Documents

PTD 1: Japanese Patent Laying-Open No. 2007-139540

PTD 2: Japanese Patent Laying-Open No. 08-271431

PTD 3: WO2005/033335

Non Patent Document

NPD 1: T. Fukuura and M. Kawasaki, “Long Range Enhancement of MolecularFluorescence by Closely Packed Submicro-scale Ag Islands”, e-Journal ofSurface Science and Nanotechnology, 2009, 7, 653

SUMMARY OF INVENTION Technical Problem

Exploiting the metal nanoparticle's localized plasmon resonance toenhance emission, as conventional, is, however, accompanied by thefollowing issue. More specifically, there are two factors for a metalnanoparticle to act to enhance emission: 1) an electric field near themetal nanoparticle is enhanced through the generation of localizedplasmon in the particle (a first factor); and 2) energy transfer from amolecule excited excites an oscillation mode of a free electron in themetal nanoparticle, which causes a radiative induced dipole in the metalnanoparticle larger than a radiative dipole of the excited molecule, andluminescent quantum efficiency per se thus increases (a second factor).In order to effectively cause, in the metal nanoparticle, the radiativeinduced dipole that is involved in the second factor, which is a largerfactor, it is required that the metal nanoparticle and the molecule tobe excited (such as fluorescent material) have a distance therebetweenwithin a range disallowing energy transfer which is a direct electrontransfer based on the Dexter mechanism to occur but allowing energytransfer based on the Förster mechanism to occur (i.e., a range of 1 nmto 10 nm). This is because the radiative induced dipole is caused basedon the theory of Förster's energy transfer (see NPD 1 above).

In general, within the range of 1 nm to 10 nm, the metal nanoparticleand the molecule to be excited with a smaller distance therebetweenfacilitate causing the radiative induced dipole and allow an increasedemission enhancement effect, whereas the metal nanoparticle and themolecule to be excited with a larger distance therebetween result inineffective localized plasmon resonance and hence a gradually reducedemission enhancement effect, and the metal nanoparticle and the moleculeto be excited with a distance therebetween exceeding the range allowingthe Förster mechanism to present energy transfer (i.e., approximately 10nm or larger in general) failed to provide a substantial emissionenhancement effect. PTDs 1-3 also describe emission enhancement methodswith a distance of 10 nm or smaller between a metal nanoparticle and amolecule to be excited to obtain an effective emission enhancementeffect.

Localized plasmon resonance via a conventional metal nanoparticle thushas such an essential issue that it has an effect in an extremelylimited range of 10 nm or smaller from a surface of the metalnanoparticle. This issue necessarily invites such an issue that littleimprovement effects can be observed in attempts applying the localizedplasmon resonance via the metal nanoparticle to a light emitting device,a photoelectric conversion device or the like aimed at improvingluminous efficiency or conversion efficiency. More specifically, a lightemitting device, a photoelectric conversion device and the like normallyhave an active layer (e.g., a light emitting layer for the lightemitting device, a light-absorbing layer for the photoelectricconversion device, and the like) having the thickness of several tens nmor larger, and even if the metal nanoparticle can be disposed adjacentto or in the active layer, a direct enhancement effect via localizedplasmon resonance can only be obtained at an extremely small portion ofthe active layer.

The present invention has been made in view of the above issue, and anobject thereof is to provide a novel plasmonic material presentingsignificantly intense plasmon resonance and also allowing plasmonresonance to have an effect over a range extended to a significantlylarge distance.

Solution to Problem

PTD 1 (see paragraphs 0010-0011) provides a theoretical explanation of arelationship between emission enhancement through localized plasmonresonance and a metal nanoparticle's particle diameter, and according tothis explanation, when a spherical silver particle having a particlediameter of approximately 500 nm is used, while luminous efficiency φ ofapproximately one is theoretically provided, in reality such a silverparticle does not present a substantial effect to enhance emission. Sucha large-size silver particle does not present a substantial effect toenhance emission because it is inferred that the silver particle has anexcessively large number of surface free electrons therein, andaccordingly, dipole-type localized plasmon observed in a typicalnanoparticle (a nanoparticle having a relatively small particlediameter) is not easily generated. It is believed, however, that if asignificantly large number of surface free electrons that the large-sizenanoparticle has therein can be effectively excited as plasmon, theplasmon would be expected to contribute to drastically more effectiveenhancement.

As a result of a diligent study, the present inventor has found that ametal-based particle assembly formed of at least a specific number oflarge-size metal-based particles having a specific shape and disposed intwo dimensions with a specific spacing therebetween can not only presentsignificantly intense plasmon resonance but also allow plasmon resonanceto have an effect over a significantly extended range (or a plasmonicenhancement effect to cover the range).

The present invention includes the following:

[1] A metal-based particle assembly comprising 30 or more metal-basedparticles separated from each other and disposed in two dimensions, saidmetal-based particles having an average particle diameter in a range offrom 200 to 1600 nm, an average height in a range of from 55 to 500 nm,and an aspect ratio, as defined by a ratio of said average particlediameter to said average height, in a range of from 1 to 8, wherein

said metal-based particles are disposed such that an average distancebetween adjacent metal-based particles may be in a range of from 1 to150 nm.

[2] The metal-based particle assembly according to item [1], whereinsaid metal-based particles are oblate particles with said aspect ratioof more than one.

[3] The metal-based particle assembly according to item [1] or [2],wherein said metal-based particles are made of silver.

[4] The metal-based particle assembly according to any one of items [1]to [3], wherein said metal-based particles are non-conductive betweenadjacent metal-based particles.

[5] A metal-based particle assembly film-layered substrate comprising: asubstrate; and a film composed of the metal-based particle assemblyaccording to any one of items [1] to [4] and layered on said substrate.

[6] The metal-based particle assembly film-layered substrate accordingto item [5], having in an absorption spectrum for a visible light regiona maximum wavelength of a peak at a longest side in wavelength, themaximum wavelength being in a range of from 350 to 550 nm.

[7] The metal-based particle assembly film-layered substrate accordingto item [5] or [6], having in an absorption spectrum for a visible lightregion a maximum wavelength of a peak at a longest side in wavelength,an absorbance at the maximum wavelength being at least one.

[8] The metal-based particle assembly film-layered substrate accordingto any one of items [5] to [7], further comprising an insulating layercovering a surface of each metal-based particle that composes said film.

[9] An optical device comprising: a light-absorbing layer having thethickness of at least 10 nm; and the metal-based particle assemblyaccording to any one of items [1] to [4] or the metal-based particleassembly film-layered substrate according to any one of items [5] to[8].

[10] An optical device comprising: a light-emitting layer having thethickness of at least 10 nm; and the metal-based particle assemblyaccording to any one of items [1] to [4] or the metal-based particleassembly film-layered substrate according to any one of items [5] to[8].

In the present specification, a metal-based particle assembly layered ona substrate will be referred to as a metal-based particle assemblyfilm-layered substrate. Furthermore, in the present specification, alight absorbing layer is a concept including a light absorbing layerthat is an active layer of a photoelectric conversion device (a solarcell device), and a light emitting layer that is an active layer of alight emitting device, for example.

Advantageous Effect of Invention

The metal-based particle assembly and metal-based particle assemblyfilm-layered substrate of the present invention, as compared with aconventional plasmonic material, present significantly intense plasmonresonance and also allow plasmon resonance to have an effect over asignificantly extended range (or a plasmonic enhancement effect to coverthe range). The metal-based particle assembly and metal-based particleassembly film-layered substrate of the present invention aresignificantly useful as an enhancement element for an optical devicesuch as a light emitting device, a photoelectric conversion device (asolar cell device) or the like, and allow an optical device therewith toprovide significant improvements of luminous efficiency or conversionefficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows SEM images (as scaled 10000 times and 50000 times) of ametal-based particle assembly film in a metal-based particle assemblyfilm-layered substrate obtained in Example 1, as observed from directlyabove.

FIG. 2 shows an AFM image of the metal-based particle assembly film inthe metal-based particle assembly film-layered substrate obtained inExample 1.

FIG. 3 shows SEM images (as scaled 10000 times and 50000 times) of ametal-based particle assembly film in a metal-based particle assemblyfilm-layered substrate obtained in Example 2, as observed from directlyabove.

FIG. 4 shows an AFM image of the metal-based particle assembly film inthe metal-based particle assembly film-layered substrate obtained inExample 2.

FIG. 5 represents absorption spectra of the metal-based particleassembly film-layered substrates obtained in Example 1 and ComparativeExamples 1 and 2.

FIG. 6 represents an absorption spectrum of the metal-based particleassembly film-layered substrate obtained in Example 2.

FIG. 7 shows an SEM image (as scaled 10000 times) of a metal-basedparticle assembly film in a metal-based particle assembly film-layeredsubstrate obtained in Comparative Example 3-1, as observed from directlyabove.

FIG. 8 shows an AFM image of the metal-based particle assembly film inthe metal-based particle assembly film-layered substrate obtained inComparative Example 3-1.

FIG. 9 represents absorption spectra of the metal-based particleassembly film-layered substrates obtained in Example 3-1 and ComparativeExample 3-1.

FIG. 10 shows SEM images (as scaled 10000 times and 50000 times) of ametal-based particle assembly film in a metal-based particle assemblyfilm-layered substrate obtained in Comparative Example 9-1, as observedfrom directly above.

FIG. 11 shows an AFM image of the metal-based particle assembly film inthe metal-based particle assembly film-layered substrate obtained inComparative Example 9-1.

FIG. 12 represents an absorption spectrum of the metal-based particleassembly film-layered substrate obtained in Comparative Example 9-1.

FIG. 13(a) schematically shows a system to measure an emission spectrumof a photoexcited light emitting device, and FIG. 13(b) is a schematiccross section view of a photoexcited light emitting device having ametal-based particle assembly film and an insulating layer.

FIG. 14 compares emission enhancement effects in photoexcited lightemitting devices of Examples 3-1 to 3-6 with emission enhancementeffects in photoexcited light emitting devices of Comparative Examples3-1 to 3-6.

FIG. 15 compares emission enhancement effects in photoexcited lightemitting devices of Examples 4-1 to 4-5 with emission enhancementeffects in photoexcited light emitting devices of Comparative Examples5-1 to 5-5 and 9-1 to 9-5.

FIG. 16 compares emission enhancement effects in photoexcited lightemitting devices of Examples 5-1 to 5-3 with emission enhancementeffects in photoexcited light emitting devices of Comparative Examples7-1 to 7-3.

DESCRIPTION OF EMBODIMENTS

<Metal-Based Particle Assembly and Metal-Based Particle AssemblyFilm-Layered Substrate>

The metal-based particle assembly of the present invention is a particleassembly comprising 30 or more metal-based particles mutually separatedand disposed in two dimensions. The metal-based particles that composethe metal-based particle assembly have an average particle diameter in arange of from 200 to 1600 nm, an average height in a range of from 55 to500 nm, and an aspect ratio, as defined by a ratio of the averageparticle diameter to the average height, in a range of from 1 to 8. Themetal-based particles that compose the assembly are disposed such thatan average distance between adjacent metal-based particles may be in arange of from 1 to 150 nm. Furthermore, the metal-based particleassembly film-layered substrate of the present invention is a substratehaving the above metal-based particle assembly layered (or carried)thereon.

As the metal-based particle assembly and metal-based particle assemblyfilm-layered substrate of the present invention are structured such thatat least a specific number of metal-based particles of a relativelylarge size having a specific shape, as described above, are disposed intwo dimensions with a specific spacing therebetween, they haveremarkable features regarding plasmon resonance, as follows.

(1) The metal-based particle assembly and metal-based particle assemblyfilm-layered substrate of the present invention can presentsignificantly intense plasmon resonance. The metal-based particleassembly of the present invention presents plasmon resonance having anintensity that is not a simple sum total of localized plasmon resonancesthat individual metal-based particles exhibit for a specific wavelength;rather, it can present an intensity larger than that. More specifically,30 or more metal-based particles each having a prescribed shape, whichare spaced as prescribed, as described above, to be closely disposed,can have the individual metal-based particles interacting with eachother to present significantly intense plasmon resonance. This isbelieved to be exhibited as the metal-based particles' localizedplasmons interact with each other.

Generally, when a plasmonic material is subjected to absorption spectrummeasurement through absorptiometry, a peak in an ultraviolet to visiblelight region is observed as a plasmon peak, and this plasmon peak'sabsorbance value in magnitude at a maximum wavelength thereof can beused to easily evaluate the plasmonic material's plasmon resonance inintensity, and when the metal-based particle assembly of the presentinvention that is layered on a substrate (a glass substrate) (i.e., ametal-based particle assembly film layered on the glass substrate) issubjected to absorption spectrum measurement through absorptiometry, itcan present for a visible light region a maximum wavelength of a plasmonpeak at a longest side in wavelength, and an absorbance at the maximumwavelength can be 1 or larger, further 1.5 or larger, and still furtherapproximately 2.

(2) Plasmon resonance has an effect over a significantly extended range(or a plasmonic enhancement effect covers the range). This feature (oreffect) is also not manifested unless metal-based particles having aprescribed shape are spaced as prescribed, as described above, to beclosely disposed, and it is believed to be exhibited as the metal-basedparticles present localized plasmons interacting with each other.Conventionally, plasmon resonance has an effect over a range that isgenerally limited to that of the Förster distance (i.e., approximately10 nm or smaller), whereas the present invention allows the range to beextended for example to approximately several hundreds nm.

Plasmon resonance having an effect over an extended range as describedabove is significantly advantageous in enhancing optical devices such aslight emitting devices, photoelectric conversion devices (solarelectronic devices) and the like. The plasmon resonance having an effectover the significantly extended range allows an active layer (e.g., alight emitting layer for the light emitting device, a light-absorbinglayer for the photoelectric conversion device, and the like) typicallyhaving the thickness of several tens nm or larger to be entirelyenhanced, thereby significantly improving the optical device'senhancement effect (such as luminous efficiency or conversionefficiency).

Furthermore, while a conventional plasmonic material needs to bedisposed to have a distance to an active layer within the range of theFörster distance, the present invention can achieve effectiveenhancement via plasmon resonance with a plasmonic material disposed ata position for example 10 nm, further several tens nm (e.g., 20 nm),still further several hundreds nm away from the active layer. This meansthat, for example for a light emitting device, the plasmonic material,or the metal-based particle assembly, can be disposed in a vicinity of alight extraction face considerably remote from a light emitting layer,and significantly improved light extraction efficiency can thus beachieved. When a conventional plasmonic material is used to produce alight emitting device, the light emitting device necessitates having theplasmonic material disposed significantly close to a light emittinglayer and hence significantly remote from a light extraction face, andaccordingly, the emitted light would have a large portion thereoftotally reflected at an interface of a variety of constituent layers ofthe light emitting device that the light passes through before itreaches the light extraction face, resulting in significantly smalllight extraction efficiency.

While the present invention employs a metal-based particle of arelatively large size that would alone be less prone to generatedipole-type localized plasmon for a visible light region, at least aspecific number of such particles, which are each required to have aprescribed shape though, are spaced as prescribed to be closely disposedto achieve significantly intense plasmon resonance and plasmon resonancehaving an effect over a significantly extended range.

Furthermore, the metal-based particle assembly and metal-based particleassembly film-layered substrate of the present invention also have sucha remarkable feature as follows.

(3) A plasmon peak's maximum wavelength presents a unique shiftdepending on the metal-based particles' average particle diameter andaverage interparticle distance. More specifically, when the metal-basedparticles have a fixed average interparticle distance while havingincreased average particle diameters, a plasmon peak at a longest sidein wavelength for the visible light region has a maximum wavelengthshifting toward a shorter side in wavelength (or blue-shifted).Similarly, when large-size metal-based particles have a fixed averageparticle diameter while having decreased average interparticle distances(i.e., when the particles are disposed more closely), a plasmon peak ata longest side in wavelength for the visible light region has a maximumwavelength shifting toward a shorter side in wavelength. This uniquephenomenon is contradictory to the Mie-scattering theory generallyaccepted regarding plasmonic materials (according to this theory, largerparticle diameters result in a plasmon peak having a maximum wavelengthshifting toward a longer side in wavelength (or red-shifted)).

It is believed that such a unique blue shift as described above is alsoattributed to the fact that the metal-based particle assembly of thepresent invention is structured with relatively large-size metal-basedparticles closely disposed with a specific spacing therebetween,followed by the metal-based particles having their localized plasmonsinteracting with each other. The metal-based particle assembly of thepresent invention (when it is layered on a glass substrate) can presentin an absorption spectrum for a visible light region, as measuredthrough absorptiometry, a maximum wavelength of a plasmon peak at alongest side in wavelength, and the maximum wavelength can be in a rangeof for example from 350 to 550 nm, depending on the metal-basedparticles' shape and interparticle distance. Furthermore, themetal-based particle assembly of the present invention typically causesa blue shift of approximately 30-500 nm (e.g., 30-250 nm) as comparedwith that having metal-based particles with a sufficiently largeinterparticle distance (for example of 1 μm).

The plasmonic material of the present invention with a plasmon peakhaving a maximum wavelength blue-shifted as compared with conventionalplasmonic material is significantly advantageous as follows. Morespecifically, while there is a strong demand for a blue luminescentmaterial (or a luminescent material of a wavelength range close thereto;the same applies hereinafter), in particular a blue phosphorescentmaterial, presenting high luminous efficiency, currently it is difficultto develop such a material with sufficiently practical usability.Applying the plasmonic material of the present invention that has aplasmon peak, for example, in the blue wavelength range to a lightemitting device as an enhancement element allows the light emittingdevice to have its luminous efficiency enhanced to a sufficient extenteven when the light emitting device uses a blue luminescent material ofrelatively low luminous efficiency. Furthermore, when it is applied to aphotoelectric conversion device (a solar cell device), it for exampleallows a resonant wavelength to be blue-shifted so that a wavelengthrange that has not been utilized by the active layer per se caneffectively be utilized to result in more efficient conversion.

Hereinafter the metal-based particle assembly and metal-based particleassembly film-layered substrate of the present invention will morespecifically be described.

The metal-based particles that compose the metal-based particle assemblyand metal-based particle assembly film-layered substrate are notspecifically restricted as long as made of a material having a plasmonpeak in an ultraviolet to visible light region in absorption spectrummeasurement through absorptiometry in the form of nanoparticles or anassembly of such particles, and the material can include, for example:noble metals such as gold, silver, copper, platinum and palladium;metals such as aluminum and tantalum; alloys containing these noblemetals or these metals; and metal compounds including these noble metalsor these metals (such as metal oxides and metal salts). Inter alia,noble metals such as gold, silver, copper, platinum and palladium arepreferable, and silver is more preferable as it is inexpensive andprovides small absorption (or has a small imaginary part of a dielectricfunction in visible light wavelengths).

The metal-based particles have an average particle diameter within arange of 200-1600 nm, and to effectively obtain the features (effects)of items (1) to (3) it falls within a range preferably of 200-1200 nm,more preferably 250-500 nm, still more preferably 300-500 nm. It shouldbe noted here that a metal-based particle of a large size having anaverage particle diameter for example of 500 nm is alone not observed toshow substantially effective enhancement via localized plasmon. Thepresent invention provides at least a prescribed number (30) of suchlarge-size metal-based particles spaced as prescribed to be closelydisposed to realize significantly intense plasmon resonance and plasmonresonance having an effect over a significantly extended range.

The average particle diameter of the metal-based particle, as referredto herein, is obtained as follows: a metal-based particle assembly(film) having metal-based particles disposed in two dimensions isobserved with an SEM from directly above to obtain an SEM image thereof,and therein ten particles are selected at random and in each particle'simage 5 tangential diametrical lines are drawn at random (note that thestraight lines serving as the tangential diametrical lines can passthrough only inside the image of the particle and one of the lines is astraight line passing through only inside the particle and drawable tobe the longest) and their average value serves as the particle'sdiameter and the 10 selected particles' respective such particlediameters are averaged to obtain the average particle diameter of themetal-based particle. The tangential diametrical line is defined as aperpendicular line connecting a spacing between two parallel linessandwiching the particle's contour (in a projected image) in contacttherewith (see the Nikkan Kogyo Shimbun, Ltd., “Particle MeasurementTechnique”, 1994, page 5).

The metal-based particle has an average height within a range of 55-500nm, and to effectively obtain the features (effects) of items (1) to (3)it falls within a range preferably of 55-300 nm, more preferably 70-150nm. The average height of the metal-based particles is obtained asfollows: the metal-based particle assembly (film) is observed with anAFM to obtain an AFM image thereof and therein 10 particles are selectedat random and measured in height and their measurements are averaged toobtain the average height.

The metal-based particle has an aspect ratio within a range of 1-8 andto effectively obtain the features (effects) of items (1) to (3) itfalls within a range preferably of 2-8, more preferably 2.5-8. Theaspect ratio of the metal-based particle is defined as a ratio of theabove average particle diameter to the above average height (i.e.,average particle diameter/average height). While the metal-basedparticle may be spherical, preferably it is oblate having an aspectratio exceeding one.

While the metal-based particle preferably has a smoothly curved surfacein view of exciting significantly effective plasmon and in particular itis more preferable that the metal-based particle be oblate having asmoothly curved surface, the metal-based particle may have a surfacewith small recesses and projections (or roughness) to some extent and inthat sense the metal-based particle may be indefinite in shape.

Preferably, the metal-based particles have variation therebetween insize as minimal as possible in view of uniformity in intensity ofplasmon resonance within a plane of the metal-based particle assembly(film). Even if there is a small variation in particle diameter, it isnot preferable that large-size particles have an increased distancetherebetween and it is preferable that particles of small size beintroduced between the large-size particles to help the large-sizeparticles to exhibit their interaction.

The metal-based particle assembly and metal-based particle assemblyfilm-layered substrate of the present invention have adjacentmetal-based particles disposed to have an average distance therebetween(hereinafter also referred to as average interparticle distance) withina range of 1-150 nm. Such closely disposed metal-based particles presentthe features (or effects) of items (1) to (3). To effectively obtain thefeatures (or effects) of items (1) to (3), the average distance iswithin a range preferably of 1-100 nm, more preferably 1-50 nm, stillmore preferably 1-20 nm. An average interparticle distance smaller than1 nm results in occurrence of electron transfer between the particlesattributed to the Dexter mechanism, which disadvantageously deactivateslocalized plasmon.

The average interparticle distance, as referred to herein, is obtainedas follows. A metal-based particle assembly (film) having metal-basedparticles disposed in two dimensions is observed with an SEM fromdirectly above to obtain an SEM image thereof, and therein 30 particlesare selected at random and for each selected particle an interparticledistance to an adjacent particle is obtained and the 30 particles' suchinterparticle distances are averaged to obtain an average interparticledistance. In obtaining an interparticle distance to an adjacentparticle, a distance to any adjacent particle, as obtained between theirsurfaces, is measured, and such measurements are averaged to obtain theinterparticle distance.

The metal-based particle assembly (film) includes 30 or more metal-basedparticles, preferably 50 or more metal-based particles. 30 or moremetal-based particles assembled together present the metal-basedparticles' localized plasmons interacting with each other effectivelyand thus present significantly intense plasmon resonance.

When the metal-based particle assembly or metal-based particle assemblyfilm-layered substrate is applied to an optical device as an enhancementelement, in light of the optical device's typical device area themetal-based particle assembly can include 300 or more metal-basedparticles, and furthermore, 17500 or more metal-based particles, forexample.

The metal-based particle assembly (film) includes metal-based particleshaving a number density preferably of 7 particles/μm² or larger, morepreferably 15 particles/μm² or larger.

The metal-based particle assembly of the present invention preferablyhas metal-based particles insulated from each other, that is, it isnon-conductive between adjacent metal-based particles (or non-conductiveas a metal-based particle assembly film). If some or all of themetal-based particles can pass/receive electrons to/from each other, theplasmon peak loses sharpness and thus resembles an absorption spectrumof bulk metal, and high plasmon resonance is not obtained, either.Accordingly, it is preferable that the metal-based particles be surelyseparated and have no conductive substance interposed therebetween.

It is preferable that a substrate that is used for the metal-basedparticle assembly film-layered substrate of the present invention be anon-conductive substrate to ensure that the metal-based particleassembly film is non-conductive. The non-conductive substrate can beformed of glass, a variety of inorganic insulating materials (SiO₂,ZrO₂, mica, and the like), and a variety of plastic materials. Inparticular, an optically transparent substrate is preferable, as, forexample, a light emitting device with the substrate applied theretoallows light to be extracted from a surface of the substrate opposite tothat having the metal-based particle assembly film layered thereon.

The metal-based particle assembly film-layered substrate preferablyfurther includes an insulating layer covering a surface of eachmetal-based particle. Such an insulating layer is preferable not only inensuring that the metal-based particle assembly film is non-conductive(between the metal-based particles), as described above, but also inapplying the substrate to an optical device. More specifically, while anoptical device such as electrical energy-driven light emitting devicehas each constituent layer with a current passing therethrough, themetal-based particle assembly film with a current passing therethroughmay result in failing to obtain a sufficient emission enhancement effectvia plasmon resonance. When the metal-based particle assembly film thatis capped by the insulating layer is applied to the optical device, theoptical device can have the metal-based particle assembly filmelectrically insulated from an adjacent constituent layer of the opticaldevice and thus have the metal-based particle assembly film without acurrent injected into the metal-based particles that compose themetal-based particle assembly film.

The insulating layer is formed of any material that is not speciallyrestricted as long as having satisfactory insulation, and it can beformed for example of spin on glass (SOG; containing organic siloxanematerial for example) and in addition thereto SiO₂, Si₃N₄ or the like.While the insulating layer is of any thickness that is not restricted aslong as ensuring desired insulation, it is better that the insulatinglayer is smaller in thickness in a range ensuring desired insulation asit is preferable that an active layer when applied to an optical device(e.g., a light emitting layer for a light emitting device, alight-absorbing layer for a photoelectric conversion device, or thelike) and the metal-based particle assembly film be closer in distance,as will be described later.

The plasmonic material (metal-based particle assembly and metal-basedparticle assembly film-layered substrate) of the present invention issignificantly useful as an enhancement element for optical devices suchas light emitting devices, and photoelectric conversion devices (solarcell devices). When the plasmonic material of the present invention isapplied to an optical device, the optical device can provide significantimprovements in luminous efficiency or conversion efficiency. As hasbeen set forth above, the plasmonic material of the present inventioncan present significantly intense plasmon resonance, and furthermore,plasmon resonance having an effect over a significantly extended range(or a plasmonic enhancement effect covering the range), and thus allowsan active layer (e.g., a light emitting layer for a light emittingdevice, a light-absorbing layer for a photoelectric conversion device,or the like) having the thickness for example of 10 nm or larger,further 20 nm or larger, still further the thickness larger than that tobe entirely enhanced. Furthermore, as has been set forth above, it alsoallows an active layer disposed at a position for example 10 nm, furtherseveral tens nm (e.g., 20 nm), still further several hundreds nm distantto be also significantly effectively enhanced.

Note that there is a tendency that, for its nature, the plasmonicenhancement effect decreases as the distance between the active layerand the metal-based particle assembly increases, so it is preferablethat the distance be smaller. The active layer and the metal-basedparticle assembly have a distance therebetween preferably of 100 nm orsmaller, more preferably 20 nm or smaller, and still more preferably 10nm or smaller.

Preferably the active layer presents an emission wavelength (for examplefor a light emitting device) or an absorption wavelength (for examplefor a photoelectric conversion device) with a maximum wavelengthmatching or close to that of the plasmon peak of the metal-basedparticle assembly film. This allows plasmon resonance to contribute to amore effectively increased emission enhancement effect. The maximumwavelength of the plasmon peak of the metal-based particle assembly filmis controllable by adjusting the film's constituent metal-basedparticles in metal type, average particle diameter, average height,aspect ratio, and/or average interparticle distance.

The above light emitting layer can for example be that formed of 1)monomolecular film including dye molecules disposed in a plane, 2) amatrix doped with dye molecules, 3) a luminescent small molecule, 4) aluminescent polymer, or the like.

Light emitting layer 1) can be obtained by applying a dyemolecule-containing liquid with spin-coating and subsequently removing asolvent. The dye molecule specifically includes by way of examplerhodamine-based dyes such as rhodamine 101, rhodamine 110, rhodamine560, rhodamine 6G, rhodamine B, rhodamine 640, and rhodamine 700 sold byExciton, a coumarin-based dye such as coumarin 503 sold by Exciton, andthe like.

Light emitting layer 2) can be obtained by applying a liquid containingdye molecules and a matrix material with spin-coating, and subsequentlyremoving a solvent. The matrix material can be a transparent polymersuch as polyvinyl alcohol and polymethyl methacrylate. The dye moleculein a specific example can be similar to that of light emitting layer 1).

Light emitting layer 3) can be obtained through dry or wet depositionincluding spin-coating, vapor deposition or the like. The luminescentsmall molecule is specifically exemplified by tris(8-quinolinolato)aluminum complex [tris(8-hydroxyquinoline) aluminum complex; Alq₃],bis(benzoquinolinolato)beryllium complex [BeBq], and the like.

Light emitting layer 4) can be obtained by a wet deposition using aluminescent polymer containing liquid such as spin-coating. Theluminescent polymer is specifically exemplified by a π-conjugatedpolymer such as F8BT [poly(9,9-dioctylfluorene-alt-benzothiadiazole)],poly(p-phenylenevinylene), polyalkylthiophene, and the like.

<Method for Producing Metal-Based Particle Assembly and Metal-BasedParticle Assembly Film-Layered Substrate>

The plasmonic material (metal-based particle assembly and metal-basedparticle assembly film-layered substrate) of the present invention canbe produced in such a method as follows:

(1) a bottom-up method to grow metal-based particles from minute seedson a substrate;

(2) a method in which a metal-based particle that has a prescribed shapeis covered with a protection layer made of an amphiphilic material andhaving a prescribed thickness, and the resultant is then subjected tolangmuir blodgett (LB) deposition to be formed in a film on a substrate;and

(3) other methods, such as a method of post-treating a thin filmobtained by vapor deposition, sputtering or the like; resist-processing;etching processing; a casting method using a liquid having metal-basedparticles dispersed therein, and the like.

It is important that method (1) includes the step of growing ametal-based particle at a significantly low speed on a substrateadjusted to have a prescribed temperature (hereinafter also referred toas the particle growth step). A production method including the particlegrowth step can provide a satisfactorily controlled layer (or thin film)of a metal-based particle assembly having 30 or more metal-basedparticles mutually separated and thus disposed in two dimensions, andhaving a shape within a prescribed range (an average particle diameterof 200 to 1600 nm, an average height of 55 to 500 nm, and an aspectratio of 1 to 8) and still preferably an average interparticle distancewithin a prescribed range (1-150 nm).

In the particle growth step, the metal-based particle is grown on thesubstrate preferably at an average height growth rate smaller than 1nm/minute, more preferably 0.5 nm/minute or smaller. The average heightgrowth rate as referred to herein can also be referred to as an averagedeposition rate or the metal-based particle's average thickness growthrate, and is defined by the following expression:

metal-based particle's average height/metal-based particle growth time(supplying time of a metal-based material).

The “metal-based particle's average height” is defined as set forthabove.

In the particle growth step, the substrate is set in temperaturepreferably within a range of 100-450° C., more preferably 200-450° C.,still more preferably 250-350° C., and particularly still morepreferably 300° C. or therearound (300° C.±approximately 10° C.).

When the production method includes the particle growth step to growmetal-based particles at an average height growth rate smaller than 1nm/minute on a substrate adjusted in temperature within the range of100-450° C., the particles are initially grown such that a suppliedmetal-based material forms a plurality of island structures, and as themetal-based material is further supplied the island structures arefurther grown and thus merged with neighboring island structures, andconsequently, metal-based particles having a relatively large averageparticle diameter are closely disposed while the metal-based particlesare each completely separated from each other to form a metal-basedparticle assembly layer. Thus a metal-based particle assembly layer canbe produced that is formed of metal-based particles controlled to have ashape within a prescribed range (in average particle diameter, averageheight, and aspect ratio) and still preferably an average interparticledistance within a prescribed range.

Furthermore, the average height growth rate, the substrate's temperatureand/or the metal-based particle growth time (the supplying time of themetal-based material) can be adjusted to also control within aprescribed range the average particle diameter, the average height, theaspect ratio, and/or the average interparticle distance of themetal-based particles grown on the substrate.

Furthermore, the production method including the particle growth stepalso allows the particle growth step to be performed such thatconditions other than the substrate's temperature and the average heightgrowth rate are selected relatively freely, and the method thus alsoadvantageously allows a metal-based particle assembly layer of a desiredsize to be efficiently formed on a substrate of a desired size.

If the average height growth rate is 1 nm/minute or larger, or thesubstrate has a temperature lower than 100° C. or higher than 450° C.,then before the island structure is grown to be large the islandstructure forms a continuum with a neighboring island structure and ametal-based assembly formed of metal-based particles mutually completelyseparated and having a large particle diameter cannot be obtained or ametal-based assembly formed of metal-based particles having a desiredshape cannot be obtained (for example, it would depart in averageheight, average interparticle distance, and aspect ratio from a desiredrange).

While the metal-based particles are grown under a pressure (in anapparatus's chamber), which may be any pressure that allows theparticles to be grown, it is normally lower than atmospheric pressure.While the pressure's lower limit is not limited to a specific value, itis preferably 6 Pa or larger, more preferably 10 Pa or larger, stillmore preferably 30 Pa or larger, as such pressure helps to adjust theaverage height growth rate within the range indicated above.

The metal-based particles can specifically be grown on a substrate inany method allowing the particles to be grown at an average heightgrowth rate smaller than 1 nm/minute, and the method can includesputtering, and vapor deposition such as vacuum deposition. Preferablesputtering is direct current (DC) sputtering as it allows a metal-basedparticle assembly layer to be grown relatively conveniently and alsofacilitates maintaining the average height growth rate smaller than 1nm/minute. The sputtering may be done in any system and it can forexample be an ion gun, or direct current argon ion sputtering to exposea target to argon ions generated by a plasma discharge and acceleratedin an electric field. The sputtering is done with a current value, avoltage value, a substrate-to-target distance and other conditionsadjusted as appropriate to grow particles at the average height growthrate smaller than 1 nm/minute.

Note that to obtain a satisfactorily controlled metal-based particleassembly layer formed of metal-based particles having a shape within aprescribed range (in average particle diameter, average height, andaspect ratio) and still preferably an average interparticle distancewithin a prescribed range, it is preferable that the particle growthstep be performed at the average height growth rate smaller than 1nm/minute and in addition thereto an average particle diameter growthrate smaller than 5 nm, and when the average height growth rate issmaller than 1 nm/minute, the average particle diameter growth rate isnormally smaller than 5 nm. The average particle diameter growth rate ismore preferably 1 nm/minute or smaller. The average particle diametergrowth rate is defined by the following expression:

metal-based particle's average particle diameter/metal-based particlegrowth time (supplying time of a metal-based material).

The “metal-based particle's average particle diameter” is defined as setforth above.

The metal-based particle growth time (or metal-based material supplytime) in the particle growth step is a period of time that at leastallows metal-based particles carried on a substrate to attain a shapewithin a prescribed range and still preferably an average interparticledistance within a prescribed range and that is smaller than a period oftime allowing the particles to depart from the shape within theprescribed range and the average interparticle distance within theprescribed range. For example, even if particle growth is performed atan average height growth rate and substrate temperature within the aboveprescribed ranges, an extremely long period of time for growth resultsin a metal-based material carried in an excessively large amount andaccordingly it will not form an assembly of mutually separatedmetal-based particles and instead form a continuous film or bemetal-based particles excessively large in average particle diameter oraverage height.

Accordingly it is necessary to grow metal-based particles for anappropriately set period of time (or to stop the particle growth step atan appropriate time), and such time can be set based for example on arelationship between an average height growth rate and a substrate'stemperature and a shape and average interparticle distance ofmetal-based particles of a metal-based particle assembly obtained, aspreviously obtained through an experiment. Alternatively a time elapsingbefore a thin film of a metal-based material grown on a substrateexhibits conduction, that is, a time allowing the thin film to be acontinuous film rather than a metal-based particle assembly film, maypreviously be obtained through an experiment and the particle growthstep may be stopped before that time is reached.

The metal-based particles are grown on a substrate preferably having assmooth a surface as possible and, inter alia, more preferably a surfacethat is smooth at the atomic level. When the substrate has a smoothersurface, thermal energy received from the substrate helps a growingmetal-based particle to merge with a surrounding, adjacent metal-basedparticle and thus grow, and thus there is a tendency to facilitateproviding a film formed of metal-based particles of a larger size.

EXAMPLES

Hereinafter, examples will be described to more specifically describethe present invention, although the present invention is not limitedthereto.

[Producing Metal-Based Particle Assembly and Metal-Based ParticleAssembly Film-Layered Substrate]

Example 1

A direct-current magnetron sputtering apparatus was used to grow silverparticles significantly slowly on a soda glass substrate to form a thinfilm of a metal-based particle assembly on the entire surface of thesubstrate to produce a metal-based particle assembly layer-layeredsubstrate under the following conditions:

gas used: argon;

pressure in chamber (sputtering-gas pressure): 10 Pa;

substrate-to-target distance: 100 mm;

sputtering power: 4 W;

average particle diameter growth rate (average particlediameter/sputtering time): 0.9 nm/minute;

average height growth rate (=average deposition rate=averageheight/sputtering time): 0.25 nm/minute;

substrate's temperature: 300° C.; and

substrate's size and shape: a square with each side having a length of 5cm.

FIG. 1 shows SEM images of a metal-based particle assembly film in theobtained metal-based particle assembly film-layered substrate, asobserved from directly above. FIG. 1(a) shows an image enlarged asscaled 10000 times and FIG. 1(b) shows an image enlarged as scaled 50000times. FIG. 2 shows an AFM image of the metal-based particle assemblyfilm in the metal-based particle assembly film-layered substrateobtained. The AFM image was obtained via “VN-8010” produced by KEYENCECORPORATION (this is also applied hereinafter). FIG. 2 shows an imagehaving a size of 5 μm×5 μm.

A calculation with reference to the FIG. 1 SEM images indicates that thepresent example provided a metal-based particle assembly configured ofsilver particles having an average particle diameter of 335 nm and anaverage interparticle distance of 16.7 nm, based on the definitionindicated above. Furthermore, from the FIG. 2 AFM image, an averageheight of 96.2 nm was obtained. From these values the silver particle'saspect ratio (average particle diameter/average height) was calculatedto be 3.48 and it can also be found from the obtained images that thesilver particles have an oblate shape. Furthermore, it can be seen fromthe SEM images that the metal-based particle assembly of the presentexample has approximately 6.25×10¹⁰ silver particles (approximately 25particles/μm²).

Furthermore, the obtained metal-based particle assembly film-layeredsubstrate had the metal-based particle assembly film connected at asurface to a tester [multimeter “E2378A” produced by Hewlett PackardCo.] to confirm conduction, and it has been found to be non-conductive.

Example 2

An aqueous silver nanoparticle dispersion (produced by Mitsubishi PaperMills, Ltd., silver nanoparticle concentration: 25% by weight) wasdiluted with pure water to have a silver nanoparticle concentration of2% by weight. Then to the aqueous silver nanoparticle dispersion 1% byvolume of a surfactant was added and sufficiently agitated andthereafter to the obtained aqueous silver nanoparticle dispersion 80% byvolume of acetone was added and sufficiently agitated at ordinarytemperature to prepare a silver nanoparticle coating liquid.

Then, the silver nanoparticle coating liquid was applied withspin-coating at 1000 rpm on a 1 mm thick soda glass substrate having asurface wiped with acetone and thereafter the substrate was left as itwas in the atmosphere for 1 minute and subsequently annealed in anelectric furnace of 550° C. for 40 seconds. A silver nanoparticle layerwas thus formed, and on the nanoparticle layer the silver nanoparticlecoating liquid was again applied with spin-coating at 1000 rpm andthereafter left as it was in the atmosphere for 1 minute andsubsequently annealed in an electric furnace of 550° C. for 40 secondsto obtain a metal-based particle assembly film-layered substrate.

FIG. 3 shows SEM images of a metal-based particle assembly film in theobtained metal-based particle assembly film-layered substrate, asobserved from directly above. FIG. 3(a) shows an image enlarged asscaled 10000 times and FIG. 3(b) shows an image enlarged as scaled 50000times. FIG. 4 shows an AFM image of the metal-based particle assemblyfilm in the metal-based particle assembly film-layered substrateobtained. FIG. 4 shows an image having a size of 5 μm×5 μm.

A calculation with reference to the FIG. 3 SEM images indicates that thepresent example provided a metal-based particle assembly configured ofsilver particles having an average particle diameter of 293 nm and anaverage interparticle distance of 107.8 nm, based on the definitionindicated above. Furthermore, from the FIG. 4 AFM image, an averageheight of 93.0 nm was obtained. From these values the silver particle'saspect ratio (average particle diameter/average height) was calculatedto be 3.15 and it can also be found from the obtained images that thesilver particles have an oblate shape. Furthermore, it can be seen fromthe SEM images that the metal-based particle assembly of the presentexample has approximately 3.13×10¹⁰ silver particles (approximately 12.5particles/μm²).

Furthermore, the obtained metal-based particle assembly film-layeredsubstrate had the metal-based particle assembly film connected at asurface to a tester [multimeter “E2378A” produced by Hewlett PackardCo.] to confirm conduction, and it has been found to be non-conductive.

Comparative Examples 1 and 2

The direct-current magnetron sputtering was done with a varieddeposition time to produce metal-based particle assembly film-layeredsubstrates for Comparative Examples 1 and 2. The metal-based particleassembly film-layered substrate of Comparative Example 1 had metal-basedparticles having approximately the same shape, aspect ratio and averageinterparticle distance as Example 1 except that the metal-basedparticles had an average height of approximately 10 nm, and themetal-based particle assembly film-layered substrate of ComparativeExample 2 had metal-based particles having approximately the same shape,aspect ratio and average interparticle distance as Example 1 except thatthe metal-based particles had an average height of approximately 30 nm.

[Measuring Absorption Spectrum of Metal-Based Particle AssemblyFilm-Layered Substrate]

FIG. 5 represents absorption spectra, as measured throughabsorptiometry, of the metal-based particle assembly film-layeredsubstrates obtained in Example 1 and Comparative Examples 1 and 2. Asindicated in a nonpatent document (K. Lance Kelly, et al., “The OpticalProperties of Metal Nanoparticles: The Influence of Size, Shape, andDielectric Environment”, The Journal of Physical Chemistry B, 2003, 107,668), an oblate silver particle as produced in Example 1 alone typicallyhas a plasmon peak around 550 nm and 650 nm for average particlediameters of 200 nm and 300 nm, respectively.

In contrast, it can be seen that Example 1's metal-based particleassembly film-layered substrate, with its constituent silver particleshaving an average particle diameter of approximately 300 nm (335 nm),nonetheless presents for a visible light region a maximum wavelength ofa plasmon peak at a longest side in wavelength, and the maximumwavelength is around approximately 450 nm, or shifted to a shorter sidein wavelength, as shown in FIG. 5. This phenomenon is manifestedcharacteristically when the silver particles are large-size particleshaving the above prescribed shape and also have the above prescribedaverage interparticle distance and are disposed significantly closely,as provided in Example 1. Such a phenomenon would not rationally beunderstandable without considering that the particles that are closelyadjacent allow their respective, internally caused plasmons to interactwith each other.

Furthermore, the plasmon peak's maximum wavelength also depends on themetal-based particles' average particle diameter. For example,Comparative Examples 1 and 2 have small average particle diameters, andaccordingly have a plasmon peak at a side considerably longer inwavelength than Example 1, with maximum wavelengths of approximately 510nm and approximately 470 nm, respectively.

Furthermore, Example 1 shows for the visible light region a maximumwavelength of a plasmon peak at a longest side in wavelength, and anabsorbance at the maximum wavelength is approximately 1.9, which issignificantly higher than Comparative Examples 1 and 2 and it can beseen therefrom that Example 1 provides a metal-based particle assemblypresenting significantly intense plasmon resonance.

FIG. 6 represents an absorption spectrum, as measured throughabsorptiometry, of the metal-based particle assembly film-layeredsubstrate obtained in Example 2. It presented for the visible lightregion a maximum wavelength of a plasmon peak at a longest side inwavelength, and the maximum wavelength was 488 nm.

Note that the absorption spectrum is obtained as follows: a metal-basedparticle assembly film-layered substrate is exposed to light of theultraviolet to visible light region incident on a back surface thereof(i.e., a side opposite to the metal-based particle assembly film) in adirection perpendicular to a substrate surface and intensity I oftransmitted light omnidirectionally transmitted toward the metal-basedparticle assembly film is measured with an integrating spherespectrophotometer. On the other hand, a substrate which does not have ametal-based particle assembly film and has the same thickness and thesame material as the substrate of said metal-based particle assemblyfilm-layered substrate is exposed at a surface thereof to the sameincident light as above in a direction perpendicular to that surface andintensity I₀ of transmitted light omnidirectionally transmitted througha side opposite to the incident surface is measured with the integratingsphere spectrophotometer. The axis of ordinate represents absorbance,which is represented by the following expression:Absorbance=−log₁₀(I/I ₀).[Producing Photoexcited Light Emitting Device and Assessing EmissionEnhancement]

Example 3-1

Silver particles were grown under approximately the same conditions asExample 1 to provide on a 0.5 mm thick soda glass substrate ametal-based particle assembly film similar to that of Example 1. Themetal-based particle assembly film had metal-based particles having thesame shape and average interparticle distance as Example 1 except thatthe metal-based particles had an average height of 66.1 nm.

Then a solution for a coumarin-based light emitting layer was appliedwith spin-coating on the metal-based particle assembly film at 3000 rpmto provide a significantly thin coumarin-based light emitting layer (onthe scale of monomolecular film) to thus obtain a light emitting device.The solution for the coumarin-based light emitting layer was prepared asfollows. Initially, a coumarin dye (Coumarin 503 from Exciton) wasdissolved in ethanol to obtain a 5 mM coumarin solution. Separately, anorganic SOG material (“OCD T-7 5500T” produced by TOKYO OHKA KOGYO CO.,LTD.) was diluted with ethanol to be 33% by volume. The organic SOGmaterial diluted to be 33% by volume, the 5 mM coumarin solution, andethanol were mixed together to have a volumetric ratio of 1:5:5 toobtain the solution for the coumarin-based light emitting layer.

Example 3-2

Silver particles were grown under the same conditions as Example 3-1 toprovide on a 0.5 mm thick soda glass substrate the metal-based particleassembly film described in Example 3-1. Thereafter immediately a SOGsolution was applied with spin-coating on the metal-based particleassembly film to have an insulating layer having an average thickness of10 nm layered thereon. For the SOG solution was used “OCD T-7 5500T”, anorganic SOG material produced by TOKYO OHKA KOGYO CO., LTD., which wasthen diluted with ethanol. “Average thickness” means average thicknessas provided on a metal-based particle assembly film having an irregularsurface, and it was measured as thickness provided when the SOG solutionwas directly applied with spin-coating on the soda glass substrate (thisis also applied to the following examples and comparative examples).When the average thickness has a relatively small value, the metal-basedparticle assembly film may have the insulating layer formed only in atrough and may not have its outermost surface entirely coveredtherewith.

Then the same solution for the coumarin-based light emitting layer asused in Example 3-1 was applied with spin-coating at 3000 rpm on theoutermost surface of the metal-based particle assembly film having theinsulating layer to provide a significantly thin coumarin-based lightemitting layer (on the scale of monomolecular film) to thus obtain alight emitting device.

Example 3-3

A light emitting device was produced similarly as done in Example 3-2except that the insulating layer had an average thickness of 30 nm.

Example 3-4

A light emitting device was produced similarly as done in Example 3-2except that the insulating layer had an average thickness of 80 nm.

Example 3-5

A light emitting device was produced similarly as done in Example 3-2except that the insulating layer had an average thickness of 150 nm.

Example 3-6

A light emitting device was produced similarly as done in Example 3-2except that the insulating layer had an average thickness of 350 nm.

Comparative Example 3-1

An aqueous silver nanoparticle dispersion (produced by Mitsubishi PaperMills, Ltd., silver nanoparticle concentration: 25% by weight) wasdiluted with pure water to have a silver nanoparticle concentration of6% by weight. Then to the aqueous silver nanoparticle dispersion 1% byvolume of a surfactant was added and sufficiently agitated andthereafter to the obtained aqueous silver nanoparticle dispersion 80% byvolume of acetone was added and sufficiently shaken and thus mixedtogether at ordinary temperature to prepare a silver nanoparticlecoating liquid.

Then, the silver nanoparticle coating liquid was applied withspin-coating at 1500 rpm on a 1 mm thick soda glass substrate having asurface wiped with acetone and thereafter the substrate was left as itwas in the atmosphere for 1 minute and subsequently annealed in anelectric furnace of 550° C. for 5 minutes to obtain a metal-basedparticle assembly film-layered substrate.

FIG. 7 shows an SEM image, enlarged as scaled 10000 times, of ametal-based particle assembly film in the metal-based particle assemblyfilm-layered substrate obtained in Comparative Example 3-1, as observedfrom directly above. FIG. 8 shows an AFM image of the metal-basedparticle assembly film in the metal-based particle assembly film-layeredsubstrate obtained in Comparative Example 3-1. FIG. 8 shows an imagehaving a size of 5 μm×5 p.m.

A calculation with reference to the FIG. 7 SEM image indicates thatComparative Example 3-1 provided a metal-based particle assemblyconfigured of silver particles having an average particle diameter of278 nm and an average interparticle distance of 195.5 nm, based on thedefinition indicated above. Furthermore, from the FIG. 8 AFM image, anaverage height of 99.5 nm was obtained. From these values the silverparticle's aspect ratio (average particle diameter/average height) wascalculated to be 2.79 and it can also be found from the obtained imagesthat the silver particles have an oblate shape. Furthermore, it can beseen from the SEM image that the metal-based particle assembly ofComparative Example 3-1 has approximately 2.18×10¹⁰ silver particles(approximately 8.72 particles/μm²).

FIG. 9 shows absorption spectra of the metal-based particle assemblyfilm-layered substrates obtained in Example 3-1 and Comparative Example3-1, as measured as described above. It can be found that Example 3-1presents in an absorption spectrum a peak wavelength (a maximumwavelength of a plasmon peak at a longest side in wavelength)blue-shifted as compared with that of the absorption spectrum ofComparative Example 3-1, and the plasmon peak at the longest side inwavelength is sharpened and an absorbance at the peak's maximumwavelength is higher.

Then, similarly as done in Example 3-1, the metal-based particleassembly film was provided thereon with a coumarin-based light emittinglayer to obtain a light emitting device.

Comparative Example 3-2

The same method as Comparative Example 3-1 was employed to provide on a1 mm thick soda glass substrate the metal-based particle assembly filmdescribed in Comparative Example 3-1. Thereafter immediately a SOGsolution was applied with spin-coating on the metal-based particleassembly film to have an insulating layer having an average thickness of10 nm layered thereon. For the SOG solution was used “OCD T-7 5500T”, anorganic SOG material produced by TOKYO OHKA KOGYO CO., LTD., which wasthen diluted with ethanol.

Then, similarly as done in Example 3-2, the metal-based particleassembly film having the insulating layer as described above wasprovided at an outermost surface thereof with a coumarin-based lightemitting layer to obtain a light emitting device.

Comparative Example 3-3

A light emitting device was produced similarly as done in ComparativeExample 3-2 except that the insulating layer had an average thickness of30 nm.

Comparative Example 3-4

A light emitting device was produced similarly as done in ComparativeExample 3-2 except that the insulating layer had an average thickness of80 nm.

Comparative Example 3-5

A light emitting device was produced similarly as done in ComparativeExample 3-2 except that the insulating layer had an average thickness of150 nm.

Comparative Example 3-6

A light emitting device was produced similarly as done in ComparativeExample 3-2 except that the insulating layer had an average thickness of350 nm.

Comparative Example 4

A light emitting device was produced similarly as done in Example 3-1except that the metal-based particle assembly film was not provided.

Example 4-1

The same method as Example 3-1 was employed to provide on a 0.5 mm thicksoda glass substrate the metal-based particle assembly film described inExample 3-1.

Then a solution for an Alq₃ light emitting layer was applied withspin-coating on the metal-based particle assembly film to form an Alq₃light emitting layer having an average thickness of 30 nm. The solutionfor the Alq₃ light emitting layer was prepared by dissolving Alq₃(Tris-(8-hydroxyquinoline) aluminum from Sigma Aldrich Co. LLC.) inchloroform to have a concentration of 0.5% by weight.

Example 4-2

The same method as Example 3-2 was employed to provide a metal-basedparticle assembly film having an insulating layer with an averagethickness of 10 nm and thereafter the same method as Example 4-1 wasemployed to form an Alq₃ light emitting layer having an averagethickness of 30 nm to obtain a light emitting device.

Example 4-3

A light emitting device was produced similarly as done in Example 4-2except that the insulating layer had an average thickness of 30 nm.

Example 4-4

A light emitting device was produced similarly as done in Example 4-2except that the insulating layer had an average thickness of 80 nm.

Example 4-5

A light emitting device was produced similarly as done in Example 4-2except that the insulating layer had an average thickness of 150 nm.

Comparative Example 5-1

The same method as Comparative Example 3-1 was employed to provide on a1 mm thick soda glass substrate the metal-based particle assembly filmdescribed in Comparative Example 3-1 and thereafter the same method asExample 4-1 was employed to form an Alq₃ light emitting layer having anaverage thickness of 30 nm to obtain a light emitting device.

Comparative Example 5-2

The same method as Comparative Example 3-2 was employed to provide ametal-based particle assembly film having an insulating layer with anaverage thickness of 10 nm and thereafter the same method as Example 4-1was employed to form an Alq₃ light emitting layer having an averagethickness of 30 nm to obtain a light emitting device.

Comparative Example 5-3

A light emitting device was produced similarly as done in ComparativeExample 5-2 except that the insulating layer had an average thickness of30 nm.

Comparative Example 5-4

A light emitting device was produced similarly as done in ComparativeExample 5-2 except that the insulating layer had an average thickness of80 nm.

Comparative Example 5-5

A light emitting device was produced similarly as done in ComparativeExample 5-2 except that the insulating layer had an average thickness of150 nm.

Comparative Example 6

A light emitting device was produced similarly as done in Example 4-1except that the metal-based particle assembly film was not provided.

Example 5-1

The same method as Example 3-1 was employed to provide on a 0.5 mm thicksoda glass substrate the metal-based particle assembly film described inExample 3-1.

Then a solution for an F8BT light emitting layer was applied withspin-coating on the metal-based particle assembly film and subsequentlyannealed with a hot plate at 170° C. for 30 minutes to form an F8BTlight emitting layer having an average thickness of 30 nm. The solutionfor the F8BT light emitting layer was prepared by dissolving F8BT (fromLuminescence Technology Corp.) in chlorobenzene to have a concentrationof 1% by weight.

Example 5-2

The same method as Example 3-2 was employed to provide a metal-basedparticle assembly film having an insulating layer with an averagethickness of 10 nm and thereafter the same method as Example 5-1 wasemployed to form an F8BT light emitting layer having an averagethickness of 30 nm to obtain a light emitting device.

Example 5-3

A light emitting device was produced similarly as done in Example 5-2except that the insulating layer had an average thickness of 30 nm.

Comparative Example 7-1

The same method as Comparative Example 3-1 was employed to provide on a1 mm thick soda glass substrate the metal-based particle assembly filmdescribed in Comparative Example 3-1 and thereafter the same method asExample 5-1 was employed to form an F8BT light emitting layer having anaverage thickness of 30 nm to obtain a light emitting device.

Comparative Example 7-2

The same method as Comparative Example 3-2 was employed to provide ametal-based particle assembly film-layered substrate having aninsulating layer with an average thickness of 10 nm and thereafter thesame method as Example 5-1 was employed to form an F8BT light emittinglayer having an average thickness of 30 nm to obtain a light emittingdevice.

Comparative Example 7-3

A light emitting device was produced similarly as done in ComparativeExample 7-2 except that the insulating layer had an average thickness of30 nm.

Comparative Example 8

A light emitting device was produced similarly as done in Example 5-1except that the metal-based particle assembly film was not provided.

Comparative Example 9-1

On a 1 mm thick soda glass substrate a 13 nm thick, thin conductivesilver film was deposited through vacuum deposition. The vapordeposition was done in a chamber having an internal pressure set at3×10⁻³ Pa. Then the substrate with the thin conductive silver filmdeposited thereon was annealed in an electric furnace of 400° C. for 10minutes to obtain a metal-based particle assembly film-layeredsubstrate.

FIG. 10 shows SEM images of a metal-based particle assembly film in theobtained metal-based particle assembly film-layered substrate, asobserved from directly above. FIG. 10(a) shows an image enlarged asscaled 10000 times and FIG. 10(b) shows an image enlarged as scaled50000 times. FIG. 11 shows an AFM image of the metal-based particleassembly film in the metal-based particle assembly film-layeredsubstrate obtained in Comparative Example 9-1. FIG. 11 shows an imagehaving a size of 5 μm×5 μm.

A calculation with reference to the FIG. 10 SEM images indicates thatComparative Example 9-1 provided a metal-based particle assemblyconfigured of silver particles having an average particle diameter of 95nm and an average interparticle distance of 35.2 nm, based on thedefinition indicated above. Furthermore, from the FIG. 11 AFM image, anaverage height of 29.6 nm was obtained. From these values, the silverparticle's aspect ratio (average particle diameter/average height) iscalculated to be 3.20.

FIG. 12 shows an absorption spectrum of the metal-based particleassembly film-layered substrate obtained in Comparative Example 9-1, asmeasured as described above. Comparative Example 9-1 presents in anabsorption spectrum a peak wavelength (a maximum wavelength of a plasmonpeak at a longest side in wavelength) located at a side longer inwavelength than that of the absorption spectrum of Example 3-1 shown inFIG. 9, and also has low absorbance at the peak wavelength.

Then the same method as Example 4-1 was employed to form an Alq₃ lightemitting layer having an average thickness of 30 nm to obtain a lightemitting device.

Comparative Example 9-2

The same method as Comparative Example 9-1 was employed to provide on a1 mm thick soda glass substrate the metal-based particle assembly filmdescribed in Comparative Example 9-1. Thereafter immediately a SOGsolution was applied with spin-coating on the metal-based particleassembly film to have an insulating layer having an average thickness of10 nm layered thereon. For the SOG solution was used “OCD T-7 5500T”, anorganic SOG material produced by TOKYO OHKA KOGYO CO., LTD., which wasthen diluted with ethanol. Then the same method as Example 4-1 wasemployed to form an Alq₃ light emitting layer having an averagethickness of 30 nm to obtain a light emitting device.

Comparative Example 9-3

A light emitting device was produced similarly as done in ComparativeExample 9-2 except that the insulating layer had an average thickness of30 nm.

Comparative Example 9-4

A light emitting device was produced similarly as done in ComparativeExample 9-2 except that the insulating layer had an average thickness of80 nm.

Comparative Example 9-5

A light emitting device was produced similarly as done in ComparativeExample 9-2 except that the insulating layer had an average thickness of150 nm.

Photoexcited light emitting devices of Examples 3-1, 3-2, 3-3, 3-4, 3-5,3-6, Examples 4-1, 4-2, 4-3, 4-4, 4-5, Examples 5-1, 5-2, 5-3,Comparative Examples 3-1, 3-2, 3-3, 3-4, 3-5, 3-6, Comparative Example4, Comparative Examples 5-1, 5-2, 5-3, 5-4, 5-5, Comparative Example 6,Comparative Examples 7-1, 7-2, 7-3, Comparative Example 8, ComparativeExamples 9-1, 9-2, 9-3, 9-4, 9-5 were assessed for emission enhancementin level, as follows. With reference to FIG. 13(a) showing a systememployed to measure the photoexcited light emitting devices' emissionspectra and FIG. 13(b) showing a schematic cross section view of aphotoexcited light emitting device, a light emitting layer 2 side of aphotoexcited light emitting device 1 was exposed to excitation light 3from a direction perpendicular to a surface of light emitting layer 2 tocause photoexcited light emitting device 1 to emit light. For anexcitation light source 4 was used a UV-LED (UV-LED375-nano produced bySOUTH WALKER, excitation light wavelength: 375 nm). Radiated wasexcitation light 3 obtained by condensing the light emitted fromexcitation light source 4 through a lens 5. An emitted light 6 fromphotoexcited light emitting device 1 in a direction of 40 degreesrelative to the optical axis of excitation light 3 was condensed by alens 7 and then transmitted through a wavelength cut-off filter 8(SCF-50S-44Y produced by SIGMA KOKI Co., LTD) to cut the light of thewavelength of the excitation light and then detected via aspectrophotometer 9 (MCPD-3000 produced by Otsuka Electronics Co.,Ltd.). FIG. 13(b) is a schematic cross section view of photoexcitedlight emitting device 1 including on soda glass substrate 100 ametal-based particle assembly film 200, an insulating layer 300, andlight emitting layer 2 provided in this order, as produced in theexamples and comparative examples.

From the spectra of the emissions detected, integrals were obtained forthe emission wavelength ranges. The respective integrals obtained fromeach emission spectrum of the photoexcited light emitting devices ofExamples 3-1, 3-2, 3-3, 3-4, 3-5, 3-6 and Comparative Examples 3-1, 3-2,3-3, 3-4, 3-5, 3-6 were divided by the integral obtained from anemission spectrum of the photoexcited light emitting device ofComparative Example 4 to obtain “emission enhancement magnification”, asrepresented in FIG. 14 along the axis of ordinate.

The respective integrals obtained from each emission spectrum of thephotoexcited light emitting devices of Examples 4-1, 4-2, 4-3, 4-4, 4-5,Comparative Examples 5-1, 5-2, 5-3, 5-4, 5-5 and Comparative Examples9-1, 9-2, 9-3, 9-4, 9-5 were divided by the integral obtained from anemission spectrum of the photoexcited light emitting device ofComparative Example 6 to obtain “emission enhancement magnification”, asrepresented in FIG. 15 along the axis of ordinate.

The respective integrals obtained from each emission spectrum of thephotoexcited light emitting devices of Examples 5-1, 5-2, 5-3 andComparative Examples 7-1, 7-2, 7-3 were divided by the integral obtainedfrom an emission spectrum of the photoexcited light emitting device ofComparative Example 8 to obtain “emission enhancement magnification”, asrepresented in FIG. 16 along the axis of ordinate.

[Producing Organic EL Device and Assessing Emission Intensity]

Example 6

Silver particles were grown under the same conditions as Example 1 toprovide on a 0.5 mm thick soda glass substrate the metal-based particleassembly film described in Example 1. Thereafter immediately a spin-onglass (SOG) solution was applied with spin-coating on the metal-basedparticle assembly film to have an insulating layer having an averagethickness of 80 nm layered thereon. For the SOG solution was used “OCDT-7 5500T”, an organic SOG material produced by TOKYO OHKA KOGYO CO.,LTD., which was then diluted with ethanol.

Then ion sputtering was employed to layer an anode electrode that is anIZO layer (thickness: 22 nm) on the insulating layer and thereafter ahole injection layer forming solution was applied with spin-coating onthe anode electrode to layer a hole injection layer having an averagethickness of 20 nm. For the hole injection layer forming solution wasused a product of PLEXTRONICS Inc. having a product name “Plexcore AQ1200” which was then diluted with ethanol to have a prescribedconcentration. The insulating layer, the anode electrode, and the holeinjection layer have a total average thickness of 122 nm (i.e., anaverage distance from a surface of the metal-based particle assemblyfilm to the light emitting layer is 122 nm).

Subsequently, a polymeric luminophor soluble in an organic solvent wasdissolved in the organic solvent at a prescribed concentration, and thissolution was applied with spin-coating on the hole injection layer toprovide a 100 nm thick light emitting layer. Then, vacuum deposition wasemployed to deposit an electron injection layer that is a NaF layer (of2 nm thick), a cathode electrode that is an Mg layer (of 2 nm thick) andan Ag layer (of 10 nm thick) in that order on the light emitting layer.The obtained device had a front surface sealed with a sealant(“XNR5516ZLV”, a UV curable resin produced by Nagase Chemtex Corp.) toobtain an organic EL device.

Comparative Example 10

An organic EL device was produced similarly as done in Example 6 exceptthat the metal-based particle assembly film was not provided.

A source meter (a source meter type 2602A produced by KeithleyInstruments Inc.) was used to apply a voltage of 15 V to the organic ELdevice of Example 6 constantly to cause the device to emit light with acurrent having a value of 2.3 mA passing between the electrodes. Theemission spectrum was measured with “CS-2000”, a spectrometric deviceproduced by Konica Minolta Co., Ltd., and the obtained emission spectrumwas integrated for a visible light wavelength range to obtain emissionintensity. The emission intensity of the organic EL device ofComparative Example 10 was also obtained, similarly as done for theorganic EL device of Example 6 (the same voltage of 15 V as the organicEL device of Example 6 was applied), except that a current having avalue of 2.7 mA was passed between the electrodes. It has been confirmedthat the resulting organic EL device of Example 6 exhibits an emissionintensity approximately 3.8 times that of the organic EL device ofComparative Example 10.

Example 7

Silver particles were grown under the same conditions as Example 1 toprovide on a 0.5 mm thick soda glass substrate the metal-based particleassembly film described in Example 1. Thereafter immediately a spin-onglass (SOG) solution was applied with spin-coating on the metal-basedparticle assembly film to have an insulating layer having an averagethickness of 30 nm layered thereon. For the SOG solution was used “OCDT-7 5500T”, an organic SOG material produced by TOKYO OHK A KOGYO CO.,LTD., which was then diluted with ethanol.

Then ion sputtering was employed to layer an anode electrode that is anIZO layer (thickness: 22 nm) on the insulating layer and thereafter ahole injection layer forming solution was applied with spin-coating onthe anode electrode to layer a hole injection layer having an averagethickness of 20 nm. For the hole injection layer forming solution wasused a product of PLEXTRONICS Inc. having a product name “Plexcore AQ1200” which was then diluted with ethanol to have a prescribedconcentration. The insulating layer, the anode electrode, and the holeinjection layer have a total average thickness of 72 nm (i.e., anaverage distance from a surface of the metal-based particle assemblyfilm to the light emitting layer is 72 nm).

Subsequently, vacuum deposition was employed to deposit on the holeinjection layer a light emitting layer that is Alq₃ of 80 nm. Then,vacuum deposition was employed to deposit an electron injection layerthat is a NaF layer (of 2 nm thick), a cathode electrode that is an Mglayer (of 2 nm thick) and an Ag layer (of 10 nm thick) in that order onthe light emitting layer. The obtained device had a front surface sealedwith a sealant (“XNR5516ZLV”, a UV curable resin produced by NagaseChemtex Corp.) to obtain an organic EL device.

Comparative Example 11

An organic EL device was produced similarly as done in Example 7 exceptthat the metal-based particle assembly film was not provided.

A source meter (a source meter type 2602A produced by KeithleyInstruments Inc.) was used to apply a voltage of 11 V to the organic ELdevice of Example 7 constantly to cause the device to emit light with acurrent having a value of 0.7 mA passing between the electrodes. Theemission spectrum was measured with “CS-2000”, a spectrometric deviceproduced by Konica Minolta Co., Ltd., and the obtained emission spectrumwas integrated for a visible light wavelength range to obtain emissionintensity. The emission intensity of the organic EL device ofComparative Example 11 was also obtained, similarly as done for theorganic EL device of Example 7 (the same voltage of 11 V as the organicEL device of Example 7 was applied), except that a current passedbetween the electrodes was adjusted to have a value of 1.1 mA. It hasbeen confirmed that the resulting organic EL device of Example 7exhibits an emission intensity approximately 2.6 times that of theorganic EL device of Comparative Example 11.

REFERENCE SIGNS LIST

1: photoexcited light emitting device, 2: light emitting layer, 3:excitation light, 4: excitation light source, 5, 7: lens, 6: emittedlight from photoexcited light emitting device, 8: wavelength cut-offfilter, 9: spectrophotometer, 100: soda glass substrate, 200:metal-based particle assembly film, 300: insulating layer.

The invention claimed is:
 1. A metal-based particle assemblyfilm-layered substrate comprising: a substrate; and a film composed of ametal-based particle assembly layered on said substrate, the metal-basedparticle assembly comprising 17500 or more metal-based particlesseparated from each other and disposed in two dimensions, saidmetal-based particles having an average particle diameter in a range offrom 200 to 1600 nm, an average height in a range of from 55 to 500 nm,and an aspect ratio, as defined by a ratio of said average particlediameter to said average height, in a range of from 1 to 8, wherein saidmetal-based particles are disposed such that an average distance betweenadjacent metal-based particles is in a range of from 1 to 150 nm, andwherein the metal-based particle assembly film-layered substrate havingin an absorption spectrum for a visible light region a maximumwavelength of a peak at a longest side in wavelength, the maximumwavelength being in a range of from 350 to 550 nm, and the substrate isformed of at least one material selected from the group consisting of aninorganic insulating material selected from SiO₂ and ZrO₂ and a plasticmaterial.
 2. The metal-based particle assembly film-layered substrateaccording to claim 1, wherein said metal-based particles are oblateparticles with said aspect ratio of more than one.
 3. The metal-basedparticle assembly film-layered substrate according to claim 1, whereinsaid metal-based particles are made of silver.
 4. The metal-basedparticle assembly film-layered substrate according to claim 1, whereinsaid metal-based particles are non-conductive with adjacent metal-basedparticles.
 5. The metal-based particle assembly film-layered substrateaccording to claim 1, wherein the absorbance at the maximum wavelengthbeing at least one.
 6. The metal-based particle assembly film-layeredsubstrate according to claim 1, further comprising an insulating layercovering a surface of each metal-based particle that composes said film.